RAYMOND: ISOTELUS GIGAS DEKAY. 251 
pygidium, 2 mm. long, shows the furrowed ribs, but less strongly, 
and pygidia 3 mm. long are quite smooth, except for the rather promi- 
nent axial lobe. The pygidia of young specimens are much smaller 
than the cephala, being both shorter and narrower. 
Summary of characters of specimens less than 5 mm. long. The 
sephalon makes up one half the entire length, and has a greater area 
than all the remainder of the shell; it is semicircular in outline, with a 
wide concave brim, and long genal spines. The eyes are very large, 
lose to the glabella and to the posterior border. The glabella is 
sonvex, prominent at the front, narrow behind, and expanding for- 
ward, with a median tubercle between the isolated lobes at the posterior 
snd. The free cheeks apparently do not meet in front. 
The thorax consists of two or three segments; the axial lobe is less 
than one fourth the total width. The pleura are flat to the fulcrum, 
which is far out, and are there turned down abruptly. Each pleuron 
pears a straight furrow, which extends beyond the fulcrum. 
The pygidium is small, semicircular, with narrow, prominent axial 
lobe, and narrow, concave border. The pleural lobes are crossed by 
listinct, furrowed ribs, and there are rings on the anterior end of the 
rxial lobe. 
Changes during development.— It will be shown in the sequel that 
some of these youthful characters are lost much more quickly than 
others. The first to be changed is the marginal position of the facial 
suture, and the most persistent is the genal spine. The ribs on the 
pygidium disappear very early, while new segments are still being 
added to the thorax. Then the glabella becomes flattened, the axial 
lobe becomes wider, the eyes relatively smaller, the genal spines shorter, 
and the cephalon more triangular and smoother. 
Outline of cephalon and pygidiwum.— The outlines of the shields, and 
their changes, were quite fully discussed by Raymond and Narraway, 
but the present more complete material has furnished some additional 
facts. In the smallest specimens so far seen, both the cephalon and 
pygidium are one half as long as wide, and regularly semicircular. 
In the cephalon, this condition persists until this shield reaches a 
length of about 4 mm., when the ratio of length to breadth begins to 
rise, and at the same time the sides of the cephalon become straighter, 
so that the general form becomes subtriangular, instead of semicircu- 
lar. ‘This change takes place rapidly, so that a cephalon 12 mm. long 
is three fourths as long as wide, and few specimens of any size have a 
higher ratio than this. Most cephala whose length is between 12 mm. 
and 30 mm. have this ratio (length divided by width), between .72 
